The activity and productivity of agricultural crops are strongly influenced by the level of salinity, i.e. by the total dissolved solids in the soil solution, which is easily determined by electric conductivity measurements.
The ability of most crops and plants to draw and extract soil water containing fertilizer by means of the roots depends on the salinity level of the soil solution. The higher the salinity or the electrical conductivity, the lower the absorbance of the soil solution by the plant. Therefore, accurate measurement of the level of salinity of the soil solution is decisive in agriculture. Namely, monitoring of the electrical conductivity of the soil solution is highly essential for setting proper treatment scheduling, for deciding when to irrigate, how much water and fertilizer to use and where they should be applied.
Devices for measuring soil properties are disclosed in U.S. Pat. No. 4,513,608, which describes a moisture sensing assembly constructed with different zones of porous material; U.S. Pat. No. 5,418,466, which discloses a sensor use for salinity measurement using an oscillating tuned circuit; US 2009/0038390, which discloses a vadose zone probe mounted on an inflatable sleeve; US 2009/0166520, which discloses an in-situ probe 5 having a plunger inserted in the probe; and U.S. Pat. No. 3,906,781, which discloses a soil probe for measuring soil pressure.
CN 1361420 relates to a probe with an electrode lead of ion meter connected with ion selective electrode sealed in a porous pottery clay cup in the bottom of probe and connected to an automatic controller via a guide pipe. The drawback of this probe 10 is the small and narrow inner space of the pottery clay which is not able to hold an EC electrode and to manage electrical conductivity tests.
DE 10058416 discloses a probe and method for determining oxygen content of interstitial water in formations containing sediment and clay. The probe has a very small outer diameter of 5-6 mm and even smaller inner diameter of 2-3 mm which allows the insertion of only a sensor. A vacuum is pulled through the side of the tube. The drawback of such a probe is as follows: the extremely narrow inner diameter of 2-3 mm is able to hold only an electrode with very little free space. The very narrow tube may be advantageous for oxygen measurement but not for measuring electrical conductivity. The reason being, that air bubbles enter the tube with soil solution and adhere to the walls of the tube and/or the electrode and in such a narrow tube the bubbles stay there without rising to the surface of the liquid. Such air bubbles distort EC measurement.
The above references all describe devices and methods for direct monitoring of ground water properties in the soil. These devices and methods may be suitable for monitoring a large number of soil properties such as pH, phosphorus, ammonium, magnesium, chlorides, nitrates, potassium, water hardness, or other soil properties but can produce inaccurate measurements or no readings at all for Electrical Conductivity (EC) since EC is highly sensitive to continuity disturbances in the ground water solution. Every change in the volume and direction of the flow of the water as a result of irrigation or rainfall may cause the creation of a temporary physical state of lack of homogeneity in the ground structure. This, in turn, includes the formation of air pockets which disturb the EC measurements. Such a situation gets worse when the water content in the ground is low, and as a result the electrodes lose contact with the 5 water in the soil.
There is also known the Mottes Soil Solution Extractor and the manual portable testing kits, which include a probe with a vacuum tube inserted therein, a porous ceramic cap or tip at the bottom and a syringe attached to the vacuum tube at the top. Soil solution is drawn into the probe through the porous ceramic cap by vacuum created by the syringe. Once the soil solution is drawn inside the probe, the solution is withdrawn into the syringe outside of the probe to be checked manually by a portable EC meter and also other testing kits. This kit allows for precise testing of soil solutions in the field as conditions change over time due to fresh soil solution that is drawn in from the soil by the manual syringe operation. The Mottes extractor is accompanied by a variety of portable soil solution testing kits for chloride, nitrate, pH and Electric Conductivity (EC).
Most of the above devices have a porous ceramic section through which ions diffuse in and out of the probe. Such devices may produce correct measurements that are indicative of certain properties of the soil despite the entrapped air in the probe. However, for EC measurements, air entrapped in the ceramic section space and/or near the EC electrodes disturbs and restricts the functioning thereof and therefore, alters the measurements to be higher than normal resistance readings (lower conductivities, or even a zero reading).
As noted above, the various available devices that conduct direct monitoring of soil properties within the ground enable the measurements of numerous soil properties except for EC, as EC is highly sensitive to continuous disturbances in the ground solution medium.
To obtain proper EC readings, there must be full and complete solution (liquid) between the sensor electrodes without any intervals or disconnections, which may result from air bubbles or air pockets. Also, when the soil is relatively dry, the solution drawn into the porous ceramic cap may lack continuity, and thus, the EC electrodes may not carry out a measurement at all and may show a read error of the value “0”.
In sensors for measuring soil properties other than EC, the method of sensing is based on the measurement of an ion concentration in the volume surrounding the sensor, and thus correct measurements can be obtained even if there is no perfect continuity of a solution surrounding the sensor. A pH sensor, for instance, examines 10 the ion concentration of hydrogen in the volume surrounding the sensor.
Thus, the prior art devices and methods are not suitable for conducting EC measurements that are reliable and accurate in all instances, as accurate EC measurements require a perfect continuity of the solution surrounding the sensor electrode.
It is an object of the present invention to overcome or eliminate the problems associated with the prior art devices.
More specifically, it is an object of the present invention to provide a probe that measures in situ, accurately and reliably the electrical conductivity (EC) of soil solutions (ground water).
The probe of the present invention is advantageous for obtaining continuous in-situ in-field measurements that are highly precise and reliable. Such measurements provide growers with in-depth information on fluctuations in the electrical conductivity levels which reflect the total dissolved solids in the soil solution. Thus, the growers benefit from water and fertilizer savings, as well as from a good harvest 25 with increased crops together with a reduction in groundwater pollution that keeps the environment clean and green.